The invention relates to dispersion strengthening methods for metals. In particular, the invention relates to artificial dispersion strengthening methods for metals.
Dispersion strengthening enhances creep rupture strength of materials. Dispersion strengthening typically occurs by introducing a fine dispersion of particles into a material, for example a metal matrix. Dispersion strengthening can occur naturally by adding material constituents that form particles when the constituents are added over their solubility limits. Alternatively, dispersion strengthening can occur artificially by adding stable particles to a material, in which these particles are not naturally occurring in the material. These particles strengthen the material and are not altered during metallurgical processing. Typically, the closer the particles, the stronger the material. The fine dispersion of close particles restricts dislocation movement, and thus strengthens the material, in particular the creep rupture strength.
FIG. 1 is an exemplary graph of creep rupture strength versus interparticle spacing (IPS), which is provided in namometers. The graph illustrates two solid curves, curves C1 and C2, that represent naturally occurring dispersion strengthened creep rupture strength data for Fe-9 and 12Cr steel at 550.degree. C. and 600.degree. C., respectively. The dashed curve, curve C3, represents a generalized relationship of creep rupture strength versus IPS, using Orowan stress .tau. (MPa) that is calculated from the equation: .tau.={Gbln(D/b)}/{2II(.lambda.-D)}, where G is shear modulus, b is Burger's vector, D is the diameter of the particle, and .lambda. is the interparticle spacing. As indicated by the curves, creep rupture strength is increased with a decrease in IPS, especially evident at low IPS (FIG. 1). A high IPS provides lower creep rupture strengths. Low creep rupture strengths are undesirable, especially in high temperature applications.
Dispersion strengthening is decreased when the spacing between the particles increases. It is believed that the spacing is increased as the particles are acted upon by physical forces to dissolve, age, convert, or transform them into different particles. These changes are due to the particles being thermally unstable and change at different temperatures or with increased time in some temperature ranges.
FIG. 2 is a graph that illustrates creep rupture strength resistance versus time for a naturally dispersion strengthened material. The curves indicate that as a metal is aged (time increases), creep rupture strength provided by naturally dispersed particles decreases due to the particles being dissolved in their metal matrix, coarsened, or otherwise changed. Any changed particles are typically spaced further apart from each other so as to reduce dispersion strengthening.
Prior artificial dispersion strengthening processes have added particles, for example particles on the order of microns in size, to molten metal, which was then cast. Particles have been added to a molten metal and optionally stirred therein. These methods did not effectively disperse the particles in the molten metal casting. The particles were too light and floated on a surface of the melt, were too heavy and sank to the melt bottom under the influence of gravity, clumped together and did not disperse, or combinations thereof. Therefore, these prior methods did not produce a dispersion strengthened metal, which is believed due to the particle size being too large.
Another prior artificial dispersion strengthening method comprised spraying particles onto a stream of molten metal. This spraying method generally requires many steps to prepare the molten metal stream, and spray. Also, this spraying process was a very lengthy process. The metal stream must flow at rates of about one (1) pound/minute (about 0.38 kilogram/minute) to provide a sufficient amount of particles in the form of a particle spray to the stream. This stream flow rate leads to long process times, especially for large articles that weigh over about 500 lb. (about 225 kilograms). Also, some of the sprayed particles did not intersect with the stream, and are thus were not utilized. Therefore, this spraying method was inefficient and timely.
Another prior artificial dispersion strengthening process attempted to add mechanically alloyed particles, for example on the order of microns in size, to a material. Mechanically alloying particles and their associated formation processes are known in the art. Typically, processes used to form mechanically alloyed particle-dispersion strengthened articles comprise powder metallurgy (PM) processes. The PM processes include, but are not limited to, hot isostatic pressing (HIP) processes, which are known in the art. PM processes have inherent size limitations in which PM production is limited to small articles (those articles that have a diameter less than about 8 inches (20 centimeters)). PM processes are impractical for dispersion strengthening of large metal articles, such as large power generation equipment including rotors for steam turbines.
Therefore, an effective dispersion strengthening method that enhances creep rupture strength for metals is needed, especially for large articles. The dispersion strengthening method should create a dispersion strengthened metal with closely spaced, uniformly dispersed, thermally stable, fine particles.